1. Field of the Invention
The invention relates to a power storage unit constituted of a power module and a casing containing the power storage module and coolant for cooling the power storage module, and the invention also relates to a vehicle having a power storage unit.
2. Description of the Related Art
Power storage units used as drive power sources or auxiliary power sources for electric vehicles and hybrid vehicles provide a high power output and therefore tend to deteriorate progressively if they are used at a temperature higher than their proper operation temperatures. To prevent such deterioration, their power storage portions need to be cooled as needed.
One of such power storage units is disclosed in Japanese Patent Application Publication No. 06-124733 (JP-A-06-124733). This power storage unit has a battery assembly constituted of a plurality of batteries connected to each other, coolant for cooling the battery assembly, and a casing containing the battery assembly and the coolant. A gas-discharge valve is provided at each battery of the battery assembly. According to this power storage unit, even if gas is produced in each battery as a result of electric decomposition of electrolyte solution when overcharging the battery, or the like, the gas is discharged via the gas-discharge valve.
Further, an agitator for agitating the coolant is provided in this power storage unit. However, when the gas is thus discharged from the battery, in some cases, the electrolyte solution and the active materials in the battery are brought into the coolant together with the discharged gas, and as a result, the electric resistance of the coolant decreases.
Further, the electric resistance of the coolant may also decrease if foreign matter such as water or impurities enters the casing from the outside.
Further, in some cases, when cooling the battery assembly, natural convection of coolant occurs in the casing, causing the coolant to collide and thus wear the casing and the battery can. In this case, metal fragment produced as a result of the wearing of the casing, thereby the electric resistance of the coolant may be reduced. Such a decrease in the electric resistance of the coolant is significant in a case where the agitator for forcibly agitating the coolant is provided as described in Japanese Patent Application Publication No. 06-124733 (JP-A-06-124733).
If the electric resistance of the coolant decreases as mentioned above, it may cause short circuits at the battery assembly, resulting in an increase in the battery temperature.